So now that there are a few hd-dpi devices any thoughts has to how to set a canvas's backing store to 1x1 to display pixels?

Ideally it would be as simple as

widthInCSSUnits = canvas.clientWidth;

heightInCSSUnits = canvas.clientHeight;

canvas.width = convertCSSUnitsToDeviceUnits(widthInCSSUnits);

canvas.height =convertCSSUnitsToDeviceUnits(widthInCSSUnits);

Unfortunately I don't know how to write 'convertCSSUnitsToDeviceUnits'

For example the new MacBook Pro with hd-dpi displays apparently have

all of the following options

2 to 1 (1440 css units scaled to 2880 device units)

1.5 to 1 (1920 css units scaled to 2880 device units)

1.7143 to 1 (1680 css units scaled to 2880 device units)

Ideally, from JS I should be able to render to a canvas at 1x1 pixels with the display regardless of which mode the system is set to by choosing a backing store size that matches. But as far as I can tell there's no API to figure that out at the moment.